One of the problems of the production of epitaxial wafers in which an epitaxial layer is deposited on the surface of a silicon single crystal wafer is the formation of a back surface deposit. Namely, the back surface of a silicon wafer made to undergo epitaxial growth in an epitaxial reactor is partially supported on a wafer support platform referred to as a susceptor. Consequently, reaction gas supplied to the epitaxial reactor during epitaxial growth may circulate to the back surface of the silicon wafer at those portions where the back surface and susceptor are not in contact.
Reaction gas that has circulated to the back surface in this manner causes epitaxial growth or polysilicon growth not only on the surface of the silicon wafer but on the back surface as well, causing a film to be deposited on the back surface of the wafer as well and resulting in the formation of a so-called back surface deposit (paragraph [0010] of Japanese Unexamined Patent Application, First Publication No. H10-275776).
Such back surface deposits may be extremely thin, having a thickness of, for example, 0.04 μm.
In addition, film thickness of an epitaxial layer of a silicon wafer is typically measured by Fourier Transform Infrared (FTIR) measurement. This method consists of measuring film thickness of an epitaxial layer based on the difference in the concentrations of impurities between the wafer and the epitaxial layer as determined with an infrared beam radiated onto the wafer surface (paragraph [0002] of Japanese Unexamined Patent Application, First Publication No. 2003-109991).